It is known with an optical readhead for reading a scale to use a light emitting diode (LED) as the primary source of illumination for the scale.
Conventionally, an LED comprises a photo-diode provided at the surface of a chip and the surface of the diode has an electrode pad connected to a wire of a driving circuit.
In the arrangement for illuminating the scale, the LED system also includes a lens for collimating the divergent light output of the LED. The lens produces on the scale an image of the electrode in the form of a darkened region. This diminishes the effective area of illumination of the scale and reduces the performance of the readhead particularly as regards tolerance to angular mis-alignment between the scale and the readhead.
A similar problem occurs with aging of the LED when areas of the LED at random emit less light and appear dark.